Light emitting diodes with smooth surface for reflective electrode

ABSTRACT

A light emitting diode comprising an epitaxial layer structure, a first electrode, and a second electrode. The first and second electrodes are separately disposed on the epitaxial layer structure, and the epitaxial layer structure has a root-means-square (RMS) roughness less than about 3 at a surface whereon the first electrode is formed.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional Application which claims the benefit of pendingU.S. patent application Ser. No. 12/120,051, filed on May 23, 2008,which claims priority to Provisional Application No. 61/041,172, filedMar. 31, 2008.

BACKGROUND

1. Field

The present disclosure relates to a light emitting diode (LED)structure, and more particularly to a LED structure with a smoothsurface for a reflective electrode.

2. Background

Light emitting diodes (LEDs) have been developed for many years and havebeen widely used in various light applications. As LEDs arelight-weight, consume less energy, and have a good electrical power tolight conversion efficiency, in some application areas, there have beenintentions to replace conventional light sources, such as incandescentlamps and fluorescent light sources, with LEDs. Such LEDs produce lightin a relatively narrow angular spread direction without side light sothat the light cannot be easily collected by optical elements in apackage. In other words, thin-film AlInGaN LEDs produce more light persteradian and photons generated therefrom can be efficiently utilizedcompared to the conventional lateral LEDs with sapphire substrateattached. However, the efficiency (Lumen/W) of the current LEDs is stillnot high enough to replace the conventional light source for generalillumination or other light applications.

Therefore, there is a need in the art to improve the structure of theLEDs so that they emit light in more efficient ways than conventionalLEDs.

SUMMARY

In an aspect of the disclosure, a light emitting diode includes anepitaxial layer structure, a first electrode, and a second electrode.The first and second electrodes are separately disposed on the epitaxiallayer structure, and the epitaxial layer structure has a RMS(root-mean-square) roughness less than 3 nm on a surface wherein thesecond electrode is formed.

In another aspect of the disclosure, a method for manufacturing a lightemitting diode includes forming an epitaxial layer structure, andseparately depositing a first electrode and a second electrode on theepitaxial layer structure. The epitaxial layer structure has a RMSroughness less than about 3 nm on a surface whereon the second electrodeis formed.

It is understood that other aspects of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein it is shown and described only exemplary aspects ofthe invention by way of illustration. As will be realized, the inventionincludes other and different aspects and its several details are capableof modification in various other respects, all without departing fromthe spirit and scope of the present invention. Accordingly, the drawingsand detailed description are to be regarded as illustrative in natureand not as restrictive.

BRIEF DESCRIPTION OF THE FIGURES

Various aspects of the present invention are illustrated by way ofexample, and not by way of limitation, in the accompanying drawings,wherein:

FIG. 1A is a cross sectional view of a vertical LED structure.

FIG. 1B is a top view of a vertical LED structure, in which a patternedn contact is shown.

FIG. 2A is a cross sectional view of a flip-chipped lateral LEDstructure with metal joints and a sub-mount.

FIG. 2B is a top view of a flip-chipped lateral LED structure with bothp and n electrodes before flip-chipped to a sub-mount.

FIG. 3 illustrates an exemplary light extraction occurring in a verticalLED structure.

FIG. 4 is a schematic diagram of a rough interface between a p-GaN layerand a silver (Ag) layer, showing that the rough interface scatters anincident light as well as couples the light into the surface plasmonmode.

FIGS. 5A-5C illustrate a process for manufacturing a vertical LED thathas a smooth surface for forming reflective electrode.

FIGS. 6A-6C illustrate a process for manufacturing a flip-chippedlateral LED structure that has a smooth surface for forming reflectiveelectrode.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various aspects of the presentinvention and is not intended to represent all aspects in which thepresent invention may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof the present invention. However, it will be apparent to those skilledin the art that the present invention may be practiced without thesespecific details. In some instances, well-known structures andcomponents are shown in block diagram form in order to avoid obscuringthe concepts of the present invention.

FIGS. 1A and 1B illustrate a cross-sectional view and a top view of avertical LED device, respectively. The vertical LED device 100, as shownin FIG. 1A, has a vertical current injection configuration, including apatterned n-type contact (or n-type electrode) 101, an n-type GaN-basedlayer 102 with a roughened surface, an active region 103, a p-typeGaN-based layer 104, a broad area reflective p-type contact (or p-typeelectrode or reflective p electrode) 105, and a thermally and/orelectrically conductive substrate 106 to support the device structuremechanically.

In the manufacturing process, the n-type GaN-based layer 102 is formedon a substrate (not shown), the active region 103 is formed on then-type GaN-based layer 102, and the p-type GaN-based layer 104 is formedon the active region 103, however, other layers may be included. Thep-type electrode 105 is directly or indirectly formed on p-typeGaN-based layer 104. The substrate on which the n-type GaN-based layer102 is formed is removed so that the patterned n-type electrode 101 canbe formed on the surface of the n-type GaN-based layer 102 that wasattached to the removed substrate. The reflective p-type electrode 105is mounted on the thermally conductive substrate 106 for mechanicalsupport.

As the n-type GaN-based layer 102 and the p-type GaN-based layer 104 areopposite to each other, together they form a pair of carrier injectorsrelative to the active region 103. Therefore, when a power supply isprovided to the LED device 100, electrons and holes will be combined inthe active region 103, thereby releasing energy in the form of light. InFIG. 1A, arrows inside the LED device 100 show that an electrical pathis generally vertically formed from the p-type electrode 105 to thepatterned n-type electrode 101. FIG. 1B shows a top view of the verticalLED of FIG. 1A, in which an n-type contact with four fingers and acrossbar is shown. It will be recognized by those of ordinary skill inthe art that the electrode pattern of the n-type contacts is not limitedto the electrode pattern as illustrated.

FIG. 2A illustrates a cross-sectional view of a flip-chipped lateral LEDdevice 200. As shown, the flip-chipped lateral LED device 200 is formedas a lateral LED device 200′ having a lateral current injectionconfiguration that is flipped over and mounted on a sub-mount substrate207 with matching metal contact pads 208. The sub-mount substrate 207may be electrically insulated or electrically conductive. The metalcontact pads 208 are electrically isolated from each other either byforming an electrically insulating sub-mount 207 or an insulatingdielectric coating formed over an electrically conducting sub-mount (notshown). The lateral LED device 200′ includes an n-type GaN-based layer201 with a roughened surface, an active region 202 formed on the n-typeGaN-based layer 201, a p-type GaN-based layer 203, a p-type electrode204, and an n-type electrode 205.

In the manufacturing process, before forming the n-type electrode, partsof the p-type electrode 204, the active region 202, and the p-typeGaN-based layer 203 are removed to allow the n-type electrode 205 to beformed on top of the n-type GaN-based layer 201. In FIG. 2A, the arrowsinside the LED device 200′ show that an electrical path is formed fromthe p-type electrode 204 to the n-type electrode 205. After the n-typeelectrode 205 is formed, the LED device 200′ is flipped over to mount onthe sub-mount substrate 207 via solders or metal interconnects 206 toform the flip-chipped lateral LED device 200.

FIG. 2B shows a top view of the LED device 200′ of FIG. 2A before beingflipped over to mount on the sub-mount substrate 207. FIG. 2B shows thatthe p-type electrode 204 has an area larger than that of the n-typeelectrode 205.

The n-type GaN-based layer, the p-type GaN-based layer, and the activelayer in the LED devices of FIGS. 1A-1B and 2A-2B are formed using aGaN-based material. When a voltage is applied to the LED devices,injected carriers (i.e., holes and electrons) recombine in the activelayers, and generate light emission. The reflective index of theGaN-based material is around 2.4 at a wavelength of 460 nm. If anincident angle of light at the interface between the GaN-based layer andthe ambient air (or other encapsulating material) is greater than acritical angle, a substantial portion of light generated inside the LEDdevice is likely to get trapped inside the LED device due tototal-internal-reflection (TIR). According to Snell's Law, the criticalangle at the GaN/air interface is about 24.6 degrees. Conventionally, toincrease the chance of light escaping from the LED device, the topsurface of the LED device is randomly roughened to break up thelimitation of the TIR.

Light extraction for an LED device will be described with reference toFIG. 3. The LED device 300 in FIG. 3 is a vertical LED device thatincludes a GaN-based material structure 310, including an n-typeGaN-based layer 302, an active region 303, a p-type GaN-based layer 304,an n-type electrode 301, and a p-type electrode 305 mounted on asubstrate 306. Reference number 320 indicates the direction of travel oflight generated inside the LED device 300. Usually, the light emittingfrom the active region 303 has about a 50/50 chance of propagatingtoward the top surface or the bottom surface of the LED device 300. Thep-type electrode (also referred to interchangeably herein as thereflective electrode) 305 is used to re-direct the light propagatingback to the top surface of the LED device 300, as shown in FIG. 3. Thereflective electrode 305 usually contains a metal, and the reflectivityof the reflective electrode 305 is made to be as high as possible toreduce reflection loss, since the light emission tends to be reflectedmultiple times before escaping the LED devices, as shown in FIG. 3.

Silver (Ag) is a suitable metal for forming a reflective electrode of anAlInGaN LED device because Ag has a high reflectivity in the wavelengthrange of interest (i.e., 400-700 nm), and this material can form ohmiccontact with a p-type GaN-based layer. Due to an epitaxial growthprocess and growth conditions used in manufacturing the GaN-basedmaterial structure, there is generally some roughness on the p-typeGaN-based layer.

FIG. 4 is an enlarged view of the p-type electrode 305 comprising Ag andthe p-type GaN-based layer 304 of FIG. 3, showing light extractioninside the LED device 300. Usually, the normal incidence of reflectanceof Ag measured from the p-type GaN-based layer side is lower than anexpected value from a simple optical model calculation because ofscattering effects 403 and surface plasmon (SP) absorption 420 resultingfrom the rough GaN/Ag interface 410, as shown in FIG. 4. The roughinterface 410 scatters the normal incident light in random directionsand reduces the specular reflection at all wavelengths. No photons aretypically lost in the scattering process because the photons willcontinue to be reflected inside the LED device and will eventuallyescape the LED device 300 to, for example, ambient air. However, thephotons coupled in the surface plasmon (SP) mode 420 typically will notbe able to escape and eventually will be lost. As result, the SPabsorption can have a significant effect on the reflectance measurement.Under experimental measurements, the strength of the SP absorptiongenerally correlates to the degree of interface roughness. Even thoughthe SP absorption peak may be below 400 nm, the width of the absorptionmay still be wide enough to significantly reduce Ag reflectance forwavelengths of around 460 nm.

The p-type electrode (e.g., Ag electrode) may be deposited on the p-typeGaN-based layer by a physical (e.g., electron-beam or thermal)evaporation process, and Ag will conform to the p-type GaN-based layersurface without voids if the deposition is performed properly. Theroughness of the p-type GaN-based layer/Ag interface is determined bythe quality of the p-type GaN-based layer. The presence of voids createdduring the deposition process or contact annealing process, however, mayincrease the interface roughness and further enhance the SP absorption.

To achieve a high reflectance value at the p-type GaN layer/Ag electrodeinterface, an LED device is provided in which the p-type GaN-based layerhas a root-mean-square (RMS) roughness less than about 3 nm to ensureappropriate smoothness of the p-type GaN-based layer/Ag electrodeinterface and to thereby minimize the SP absorption.

In a variation, a method for manufacturing the p-type GaN-based layerwith a smooth surface is provided, such that the SP absorption can bereduced to a maximum extent.

FIGS. 5A-5C and 6A-6C illustrate manufacturing processes for a verticalLED device 500 and a flip-chipped lateral LED device 600, respectively.The processes of FIGS. 5A and 6A basically follow similar manufacturingsteps, except with regard to the formation of an n-type electrode andfinal mounting.

In FIG. 5A, an n-type GaN-based layer 502 is formed on a substrate, suchas a sapphire substrate 501. Above the n-type GaN-based layer 502, anactive layer 503 and a p-type GaN-based layer 504 are formed. In anexample, the RMS roughness of the surface of the p-type GaN-based layer504 that will interface with a p-type electrode 505, shown in FIG. 5B,is preferably controlled to be less than about 3 nm. In an example, thep-type GaN-based layer 504 is formed by MOCVD (Metal-Organic ChemicalVapor Deposition) using metal-organic compound such as trimethyl gallium(TMGa), trimethyl indium (TMIn), trimethyl aluminum (TMAl), and ammonia,hydrogen, nitrogen as well as dopant precursors for silicon andmagnesium in a reactor chamber with controlled pressure and temperature.Furthermore, the growth temperature in the deposition process ispreferably greater than about 950° C. and the growth rate is preferablyless than about 150 Å/min.

After depositing the p-type GaN-based layer with RMS roughness less thanabout 3 nm, the p-type electrode 505 is formed on the p-type GaN-basedlayer 504, as shown in FIG. 5B. Alternatively, prior to depositing thep-type electrode 505, a transparent ohmic contact layer (not shown) maybe formed on the p-type GaN-based layer 504. The transparent ohmiccontact layer may be formed by, for example, electron-beam evaporation,sputtering, MOCVD, etc., with doped metal oxides, such as indium tinoxide (ITO) or aluminum doped zinc oxide (AZO). Similarly, when thetransparent ohmic contact layer is presented, the RMS roughness of thetransparent ohmic contact layer may be controlled to be less than about3 nm regardless of the RMS roughness of the p-GaN surface.

As described above, the p-type electrode 505 of FIGS. 5B and 5C is areflective layer for reflecting light emitted downwardly back to the topsurface of the LED device, as shown in FIGS. 5B and 5C. Exemplary metalsused in forming the p-type electrode 505 include Ag, Pt, Ni, Cr, Ti, Al,Pd, Ru, Rh, Mo, and their alloys.

In a variation, after the p-type electrode 505 is formed, the substrate501 is removed from the n-type GaN-based layer 502 to allow an n-typeelectrode 506 to be formed on the surface of the n-type GaN-based layer502 that was attached to the substrate 501, as shown in FIG. 5C. Thesurface of the n-type GaN-based layer 502 on which the n-type electrode506 is formed is roughened according to a conventional roughening methodto minimize the total internal reflection (TIR) effect and enhance lightextraction efficiency. Also, as shown in FIG. 5C, the p-type electrode505 may be mounted on a sub-mount substrate 508 for a mechanicalsupport. The sub-mount substrate 508 may include similar materials tothose used in the substrate 501. That is, the sub-mount substrate 508may be selected from one or more of the followings: metals, such as Cu,Mo, W, and Al, or their alloys; semiconductor materials, such as Si,GaAs, GaP, InP, and Ge; and/or ceramics, such as Al₂O₃ and AlN.

FIGS. 6A-6C will now be described in further detail. Similar to FIG. 5A,in FIG. 6A, an n-type GaN-based layer 602, an active layer 603, and ap-type GaN-based layer 604 are formed on a substrate, such as a sapphiresubstrate 601. As described above with reference to FIG. 5A, the p-typeGaN-based layer 503 may be formed by a MOCVD (Metal-Organic ChemicalVapor Deposition) using metal-organic compound such as trimethyl gallium(TMGa), trimethyl indium (TMIn), trimethyl aluminum (TMAl), and ammonia,hydrogen, nitrogen as well as dopant precursors for silicon andmagnesium in a reactor chamber with controlled pressure and temperature.The growth temperature in the deposition process is preferably greaterthan about 950° C. and the growth rate is preferably less than about 150Å/min. Furthermore, the RMS roughness of the p-type GaN-based layer maybe, for example, less than about 3 nm.

Next, in FIG. 6B, a transparent ohmic contact layer 609 and a reflectivep-type electrode 605 are formed on the p-type GaN-based layer 604. Asdescribed above, the transparent ohmic contact layer 609 is optional anddoes not limit the scope of the invention. The RMS roughness of thesurface in direct contact with reflective electrode 605, for example,the p-GaN surface or the transparent ohmic contact layer surface, may beless than 3 nm.

As a flip-chipped lateral LED device 600, FIG. 6C shows that parts ofthe reflective p-type electrode 605, the transparent ohmic contact layer609, and the p-type GaN-based layer 604 are etched away to allow ann-type electrode 606 to be formed on top of the n-type GaN-based layer602, as shown in FIG. 6C. The LED structure made by this process is thenflipped over and mounted on a sub-mount substrate 608 with matchingmetal contact pads 611 via solder joints or metal interconnects 610. Themetal contact pads 611 are electrically isolated from each other eitherby an electrically insulating sub-mount 608 or an insulating dielectriccoating formed over an electrically conducting sub-mount (not shown).

Example embodiments in accordance with aspects of the present inventionhave now been described in accordance with the above advantages. It willbe appreciated that these examples are merely illustrative of aspects ofthe present invention. Many variations and modifications will beapparent to those skilled in the art.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. All structural andfunctional equivalents to the elements of the various aspects describedthroughout this disclosure that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the claims. Moreover,nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims. No claim element is to be construed under the provisions of 35U.S.C. §112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

1. A method for manufacturing a light emitting diode, comprising:forming an epitaxial layer structure; and depositing a first electrodeand a second electrode on the epitaxial layer structure, wherein theepitaxial layer structure has a root-means-square (RMS) roughness lessthan about 3 nm on at a surface whereon the second electrode is formed.2. The method of claim 1, wherein the second electrode is formed ofdepositing metals including at least one selected from a groupconsisting of Ag, Pt, Ni, Cr, Ti, Al, Pd, W, Ru, Rh, Mo, and theiralloys.
 3. The method of claim 1, wherein the epitaxial layer structureincludes a first and second epitaxial layers, wherein the firstepitaxial layer is between a first electrode and the second epitaxiallayer, and the second epitaxial layer is between the second electrodeand the first epitaxial layer.
 4. The method of claim 1, wherein thesecond electrode is formed on the second epitaxial layer and a surfaceof the second epitaxial layer where the second electrode is formed has aroot-means-square (RMS) roughness less than about 3 nm.
 5. The method ofclaim 3, wherein the first epitaxial layer is an n-type epitaxial layerand the second epitaxial layer is a p-type epitaxial layer.
 6. Themethod of claim 5, wherein the first electrode is an n-type electrodeand wherein the second electrode is a p-type electrode.
 7. The method ofclaim 5, further comprises forming an active layer between the n-typeepitaxial layer and the p-type epitaxial layer.
 8. The method of claim3, further comprising depositing a transparent ohmic contact layerformed between the second epitaxial layer and the second electrode. 9.The method of claim 8, wherein a surface of the transparent ohmiccontact layer adjacent to the second electrode has a root-means-square(RMS) roughness less than about 3 nm.
 10. The method of claim 3, whereinthe second epitaxial layer is formed via a MOCVD (Metal-Organic ChemicalVapor Deposition) using a metal-organic compound solution.
 11. Themethod of claim 10, wherein the second epitaxial layer is formed at atemperature no less than 950° C. at a growth rate no more than 150Å/mim.
 12. The method of claim 1, wherein the first and secondelectrodes are deposited on either sides of the epitaxial layerstructure.
 13. The method of claim 12, wherein the light emitting diodeis a vertical light emitting diode device.
 14. The method of claim 12,wherein the second electrode is mounted on a substrate.
 15. The methodof claim 14, wherein the substrate is selected from a group consistingof a metal, a semiconductor material, and a ceramic.
 16. The method ofclaim 15, wherein the metal includes at least one selected from a groupconsisting of Cu, Mo, W, and Al, wherein the semiconductor materialincludes at least one selected from a group consisting of Si, GaAs, GaP,InP, and Ge, and wherein the ceramic includes at least one selected froma group consisting of Al₂O₃ and AlN.
 17. The method of claim 1, whereinthe first and second electrodes are deposited on one side of theepitaxial layer structure.
 18. The method of claim 17, wherein the lightemitting diode is a flip-chipped lateral light emitting diode device.19. The method of claim 17, wherein the epitaxial layer structureincludes a p-type epitaxial layer and a n-type epitaxial layer, whereinthe n-type epitaxial layer is between the first electrode and the p-typeepitaxial layer, and the p-type epitaxial layer is between the secondelectrode and the n-type epitaxial layer, and wherein the firstelectrode is located on the n-type epitaxial layer.
 20. The method ofclaim 17, further comprising mounting the first and second electrodes ona substrate.
 21. The method of claim 20, wherein the first and secondelectrodes are mounted on the substrate via one of a solder weld and ametal interconnect.
 22. The method of claim 20, wherein the substratehas an insulating coating thereon.
 23. The method of claim 20, whereinthe substrate is formed by depositing one selected from a groupconsisting of a metal, a semiconductor material, and a ceramic.
 24. Themethod of claim 23, wherein the metal includes at least one selectedfrom a group consisting of Cu, Mo, W, and Al, wherein the semiconductormaterial includes at least one selected from a group consisting of Si,GaAs, GaP, InP, and Ge, and wherein the ceramic includes at least oneselected from a group consisting of Al₂O₃ and AlN.